Process of producing microporous material



UNITED STATES PATENT OFFICE PROCESS OF PRODUCING MICROPOBOUS MATERIAL Erie Maurice OConor Honey and Charles Rupert Hardy. Dagenham Dock, England, assignors to The Electric Storage Battery Co., a corporation of New Jersey No Drawing. Applicationiletober is, 194:, So-

ri9a4l2No. 506,600. In Great Britain mm 11, 1

6 Claims. (01. 18-48) This invention relates to the production of shaped material upon an endless conveyor micro-porous material, which can be advantathrough a tunnel or channel in which the strip geously used as separators or diaphragms more is subjected to heat, preferably the radiant heat particularly between the plates of electric acof lamps, in order to evaporate the solvent which cumulators, and has for its object to provide an is then recoveredimproved process for producing such material, The invention further comprises passing the separators or diaphragms. The requisites of a material containing starch as the pore formseparator for an electric accumulator are a high ing agent and from which the solvent has been degree of micro-porosity (to allow maximum difremoved into sulphuric acid at 90 to 100 C. to fusion of the electrolyte and free movement of lo soluhiiize the starch, then into wash water, with ions) and the requisite strength or toughness for a final dryin handling and to maintain shape when wet. In The invention further comprises passing the addition, the separator should be reasonably inmaterial containing starch to form the desired ert to the electrolyte and have some degree of pores and fromwhich solvent has been removed. flexibility. It is desirable that the separator l first into boiling water in order to swell the starch should last in service at least ,as long as the actherein, (the material increasing in dimensions cumulator plates. during such swelling), then into sulphuric acid In accordance with our invention, we produce at 90 to 100 C. which solubilises the .starch and a dough by incorporating in polyvinyl choride, causes the strip to shrink from its swollen condia thermoplastic polymer containing saturated 20 tion, then into boiling wash water, the strip bealiphatic structural chains, fine particles of a ing finally dried at bout 70 C., and cut up as substance which can be subsequently removed desired. 7 v therefrom and which then leave micro pores in The invention further comprises treating the the polyvinyl chloride, and a solvent for the material before treatment with sulphuric acid, polyvinyl chloride which acts as a temporary with caustic alkali at 90 to 100 C., the said plasticiser thereof (in that it softens and discaustic alkali treatment following or taking the perses the polyvinyl chloride and is readily replace of the boiling water treatment. movable) and which has no deleterious eflect The inventio further comprises treating the upon the said particles, the resulting dough" material with a wetting-out agent, .such as sulbeing formed into desired shapes, the solvent rephonated oleic acid, prior to final drying. Such moved, and the dough treated for the removal of treatment causes the surfaces of the polyvinyl the particles so as to leave micropores in the chloride to be readily wettable' by electrolyte. solid polyvinyl chloride, and dried. There may Thus the electrical resistance of the porous me.- be a treatment 0! the micro porous material with terial when used as a diaphragm or separator for a suitable wetting-out agent. accumulators is appreciably reduced. The invention further c mpr s the thorough In one convenient application of our invention. mixing with the polyvinyl chlori e 1' r h we use maize starch which has been suitably whi h may h been P i ly dried and if dried and if necessary screened to remove lumps necessary screened to remove y lumps large or large particles. and we mix 75 lbs. thereof P le-Maud a solvent. such as methyl cy l 40 with 15 lbs. of a thermoplastic polymer containhexenone. which acts as a temporary plasticiser. ing saturated aliphatic structural chains such as 01' h polyvinyl chloride, he mixing being efl'eotpolyvinyl chloride in the form of unplasticised ed at an elevated emp r ure (say a temperapowder and 29 lbs. or methyl cyclohexanone. ture of 80 'to 100 C.) in a closed vessel fitted The mixer is heated to about100C. and is fitted with a reflux condenser. the hot dou h ein h n with a reflux condenser. After about aominutes moulded, extruded or rolled to the desired shape. mixing, the mixer and contents are cooled to The invention further comprises extruding the C. or thereabouts and the dough transferred to dough from a die box heated to a temperature at a die box heated to about 80 C. fitted with a which the dough is sufliciently plastic, through a hydraulic plunger. There is a suitably shaped suitably shaped die rose or orifice on to a travelso die-rose or orifice on the box. The extruded ling belt or conveyor moving at such a speed strip is delivered on to a. belt conveyor travelling that stretching and consequent narrowing of the at such a speed, which may be about 10 feet per extruded strip is avoided, as is also cockling due minute, that it neither draws out nor cockles to insufllcient belt speed. the strip. The latter after a certain length, say The invention further comprises passing the to six feet, of travel on the conveyor may be trans- Ierred to a further conveyor which passes it through a tunnel or channel with radiant heat lamps therein maintaining the temperature of the material at about 100 to 130 C. to effect evaporation of the solvent, which is then recovered. The length of the tunnel or channel, or of the part thereof in which heat is applied to the material, will be "determined by the time taken to complete the evaporation of the solvent,

to 100 C. which digests the starch, then through boiling wash water for about 30 minutes to re move the acid and then finally for several hours through a wetting-out agent. It is finally air dried at about 70 C. and cut to desired dimensions.

It will be appreciated that the starch granules swell considerably when the strip is immersed in boiling water or alkali for any length of time, and the strip increases in dimensions. The shrinkage in the subsequent treatment depends .upon the temperature at which it is carried out and at 90 to 100 C. we find the strip will return to about its original dimensions. The swelling treatment with boiling water and/or the alkali treatment improves the performance-of the diaphragm as an accumulator separator.

Starch is a. suitable material to give micro porosity by its subsequent removal, because any particular species, suchas maize starch, possesses fairly uniform particle size, and the particles are not seriously altered in the mixing of the dough and its fabrication into diaphragms or separators.

The choice of solvent is governed by its ability to disperse and soften the: polyvinyl chloride so as to produce a workable dough at a convenient working temperature as before indicated, and by its capacit for ready removal and recovery from the extruded, or shaped material. Examples of suitable solvents are cvclohexanone, methyl cyclohexanone and chlorobenzene.

Polyvinvl chloride is usua ly in commerce mixed with plasticisers to reduce its brittleness, but we have found thatduring mixing with starch, the solvent gives the necessary properties to unplas ticised polyvinyl chloride to enable it to coat the starch granules and to flow freely, and though 4 elastic to a plastic condition, being a thermoplastic material-these terms being used to refer to the fact that the properties of the material above and below a given temperature are different in terms of elasticity and plasticity.

The micro porous polyvinyl chloride, which constitutes our material, diaphragms or separators. may if desired be loaded with wood floor orother suitable filler for cheapness of production. The material may also be coloured by the addition of suitable colouring ingredients.

Our material when made into separators for electric accumulators is more inert to the electrolyte and oxidising materials, such as nascent" oxygen, than wood separators and has a much longer life. It is also stronger than wood for separators, wood being weak across the grain, the micro porosity of our product can be controlled, whereas the micro porosity of wood is determined by the spatial arrangement of its cellulose.

The process of manufacture of our material in the form of diaphragms or separators for electric accumulators is much simpler and more economical than the usual process of manufacture of micro porous rubber separators, which have to bevulcanised and require considerable preparatory treatment prior to the mixing in of the starch. Many micro porous rubber separators are very brittle and diflicult to handle.

,The fact that we can produce the required sections of our material in continuous lengths by extrusion is a great advantage from the point of view of large scale economical manufacture.

Waste or spoilt material can be reprocessed, which is an advantage.

,We claim:

l. A process for the production of micropcrous material comprising the steps of mixing approximately one part by weight of dry, finely divided particles of polyvinyl chloride, approximately five parts by weight of dry, finely divided starch, and approximately two parts by weight of a solvent of the group consisting of cyclohexanone, methyl cyclohexanone and chlorobenzene, and thereby forming a dough in which said solvent acts as a temporary plasticizer for the coating of the starch particles, extruding the dough as a sheet, removing the plasticizer by evaporation, passing the extruded sheet through water at approximately boiling temperature to swell the starch particles thereby to increase the dimensions of the sheet, passing the swollen sheet through hot acid to digest the starch and to shrink the sheet to approximately its original dimensions, and then the product is brittle after the evaporation of the solvent therefrom, it acquires a degree of flex ibility and toughness when the starch is removed and the strip or article becomes micro-porous. A

- solvent has the advantage that it acts as a temporary plasticiser in that it disperses and softens the polyvinyl chloride during the production of the strip of diaphragm, after which it can be "removed, when flexibility of the product is obtained due to its porosity. If exceptional flexibility is required, so as to enable, for example, the micro-porous material to be used as an envelope to enclose a plate of an accumulator, then one of. the usual plasticisers, such as tri-cresyl phosphate, may be incorporated in the mixture in addition to the solvent.

It is advantageous to carry out the processim' of the polyvinyl chloride, except its final drying,

at temperatures exceeding 78 C. At about that temperature polyvinyl chloride changes from an washing and drying the sheet. V

2. A process for the production of microporous material comprising the steps of forming an extrudable dough from polyvinyl chloride and finely divided particles of starch by mixing them with a solvent for the polyvinyl chloride which acts as a temporary plasticizer thereof to cause the polyvinyl chloride to coat the starch particles, extruding the dough as a strip, removing the solvent by evaporation, treating the extruded strip with water at approximately boiling temperature for a time adequate to cause swelling of the starch particles materially to increase the dimensions of the strip, passing the swollen strip through hot acid for a time adequate to digest the starch and to allow the strip to return to approximately its original dimensions, and then washing and drying the strip.

divided particles of starch by mixing them with a solvent for the polyvinyl chloride which acts as a temporary plasticizer thereof to cause the polyvinyl chloride to coat the starch particles. forming the dough into a desired shape, removing the solvent by evaporation, treating the shaped material with a hot liquid capable of swelling the starch particles to increase the dimensions of the shaped material, passing the swollen shaped material through hot acid to digest the starch and to allow the shaped material to return to approximately its original dimensions, and then washing and drying the shaped material.

4. A process for the production of microporous material comprising the steps of forming a dough from polyvinyl chloride and finely divided particles of starch with a solvent for the polyvinyl chloride which acts as a temporary plasticizer thereof for coating said starch particles, extrud ing the dough as a strip, removing the plasticizer by evaporation, passing the extruded strip through an aqueous solution of an alkali at approximately boiling temperature to swell the starch particles thereby to increase the dimensions of the strip, passing the swollen strip through hot acid which digests the starch for shrinkage of the strip to approximately its original dimensions, washing the strip, immersing it in a wetting-cut agent, and then drying the strip.

5. A process for the production of microporous separators for battery plates comprising the steps of mixing with finely divided particle of polyvinyl chloride a pore-forming agent comprising finely divided starch and a solvent in suflicient quantity to act as a temporary plasticizer, working the material thoroughly to coat the starch particles with said polyvinyl chloride and to form an extrudable dough, extruding the dough as a sheet, removing the solvent by evaporation, the extruded sheet then being relatively brittle, treating the brittle sheet with water at approximately its boiling temperature for a time interval suincient for adequate water absorption by the finely divided starch to cause the particles thereof to swell and to stretch the polyvinyl chloride coating, removing the starch by digesting the particles in 5% sulfuric acid at a temperature in the range of 90 C. to 100 C., and then washing and drying the sheet which due to its porosity is materially less brittle than before removal of said starch.

6. A process for the production of microporous separators for battery plates comprising the steps 6 of mixing with finely divided particles of polyvinyl chloride a pore-forming agent comprising finely divided starch and a solvent in sumcient quantity to act as a temporary plasticizer, said solvent being selected from the group consisting of cyclohexane, methyl cyclohexanone, and

chlorobenzene, working the material thoroughly to coat the starch particles and to form an extrudable dough, forming the dough into a desired shape, removing the solvent by evaporation, the shaped dough then being relatively brittle, treating the brittle shaped dough with water at approximately its boiling temperature for a time interval sufllcient for adequate water absorption by the finely divided starch to cause the particles thereof to swell and to stretch the polyvinyl chloride coating to render the shaped dough less brittle and "more porous after removal of the starch, removing the starch by digesting ERIC MAURICE OCONOR HONEY. CHARLES RUPERT HARDY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 788,173 Cross Dec. 20, 1904 941,159 Militz Nov. 23, 1909 1,332,348 Macdonald Mar. 2, 1920 1,929,453 Semon Oct. 10, 1933 2,011,160 Plepp Aug. 13, 1935 2,023,204 Monters et al. Aug. 20, 1932 2,175,798 Hauser Oct. 10, 1939 2,181,299 Burges Nov. 28, 1939 2,189,889 Engel Feb. 13, 1940 2,315,366 Daley et al Mar. 30, 1943 2,371,868 Berg et al Mar. 20, 1945 2,372,669 Haney et al Apr. 3, 1945 2,372,695 Taylor Apr. 3, 1945 FOREIGN PATENTS Number Country Date 228,844 Great Britain June 8, 1925 402,055 Great Britain Apr. 8, 1932 OTHER REFERENCES Ser. No. 268,117, Engel (A. P. C), published Apr. 2'7, 1943.

Certificate of Correction Patent No. 2,542,527 February 20, 1951 ERIC MAURICE OCONOR HONEY ET AL.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column 5, line 30, for the words wetting-cut read wettwhzg-out;

and that the said Letters Patent should be read as corrected above, so that the same may conform to the record of the case in the Patent Oflice.

Signed and sealed this 5th day of- J une, A. D. 1951.

THOMAS E. MURPHY,

Assistant of Patents. 

1. A PROCESS FOR THE PRODUCTION OF MICROPOROUS MATERIAL COMPRISING THE STEPS OF MIXING APPROXIMATELY ONE PART BY WEIGHT OF DRY, FINELY DIVIDED PARTICLES OF POLYVINYL CHLORIDE, APPROXIMATELY FIVE PARTS BY WEIGHT OF DRY, FINELY DIVIDED STARCH, AND APPROXIMATELY TWO PARTS BY WEIGHT OF A SOLVENT OF THE GROUP CONSISTING OF CYCLOHEXANONE, METHYL CYCLOHEXANONE AND CHLOROBENZENE, AND THEREBY FORMING A DOUGH IN WHICH SAID SOLVENT ACTS AS A TEMPORARY PLASTICIZER FOR THE COATING OF THE STARCH PARTICLES, EXTRUDING THE DOUGH AS A SHEET, REMOVING THE PLASTICIZER BY EVAPORATION, PASSING THE EXTRUDED SHEET THROUGH WATER AT APPROXIMATELY BOILING TEMPERATURE TO SWELL THE STARCH PARTICLES THEREBY TO INCREASE THE DIMENSIONS OF THE SHEET, PASSING THE SWOLLEN SHEET THROUGH HOT ACID TO DIGEST THE STARCH AND TO SHRINK THE SHEET TO APPROXIMATELY ITS ORIGINAL DIMENSIONS, AND THEN WASHING AND DRYING THE SHEET. 